A mutex is a lock that is used to serialize all threads through a piece
of code by calling Tcl_MutexLock and Tcl_MutexUnlock.
If one thread holds a mutex, any other thread calling Tcl_MutexLock will
block until Tcl_MutexUnlock is called. A thread can deadlock
on itself if it tries to lock the mutex twice.
Tcl_MutexLock and Tcl_MutexUnlock
procedures are defined as empty macros if not compiling with threads enabled.

A condition variable is used as a signaling mechanism:
a thread can lock a mutex and then wait on a condition variable
with Tcl_ConditionWait. This atomically releases the mutex lock
and blocks the waiting thread until another thread calls
Tcl_ConditionNotify. The caller of Tcl_ConditionNotify should
have the associated mutex held by previously calling Tcl_MutexLock,
but this is not enforced. Notifying the
condition variable unblocks all threads waiting on the condition variable,
but they do not proceed until the mutex is released with Tcl_MutexUnlock.
The implementation of Tcl_ConditionWait automatically locks
the mutex before returning.

The caller of Tcl_ConditionWait should be prepared for spurious
notifications by calling Tcl_ConditionWait within a while loop
that tests some invariant.

The Tcl_GetThreadData call returns a pointer to a block of
thread-private data. Its argument is a key that is shared by all threads
and a size for the block of storage. The storage is automatically
allocated and initialized to all zeros the first time each thread asks for it.
The storage is automatically deallocated by Tcl_FinalizeThread

All of these synchronization objects are self initializing.
They are implemented as opaque pointers that should be NULL
upon first use.
The mutexes and condition variables are
cleaned up by process exit handlers. Thread local storage is
reclaimed during Tcl_FinalizeThread.

The API to create threads is not finalized at this time.
There are private facilities to create threads that contain a new
Tcl interpreter, and to send scripts among threads.
Dive into tclThreadTest.c and tclThread.c for examples.